1. Field of the Invention
The present invention relates generally to wireless communication systems and, more specifically, to the transmission and reception of Physical DownLink Control CHannels (PDCCHs).
2. Description of the Art
A conventional communication system includes a DL that conveys transmission signals from transmission points such as Base Stations (BS or NodeBs) to User Equipments (UEs) and an UpLink (UL) that conveys transmission signals from UEs to reception points such as NodeBs. A UE, also known as a terminal or a mobile station, is fixed or mobile and is a cellular phone or a Personal Computer device, for example. A NodeB, also known as an access point, is generally a fixed station.
DL signals include data signals carrying information content, control signals carrying DL Control Information (DCI), and Reference Signals (RSs), which are also known as pilot signals. A NodeB transmits data information or DCI to UEs through respective Physical DL Shared CHannels (PDSCHs) or PDCCHs.
UL signals also include data signals, control signals and RSs. A UE transmits data information or UL Control Information (UCI) to a NodeB through a respective Physical Uplink Shared CHannel (PUSCH) or a Physical Uplink Control CHannel (PUCCH).
A PDSCH transmission to a UE or a PUSCH transmission from a UE is in response to dynamic scheduling or to Semi-Persistent Scheduling (SPS). With dynamic scheduling, a NodeB conveys to a UE a DCI format through a respective PDCCH. With SPS, a PDSCH or a PUSCH transmission is configured to a UE by a NodeB through higher layer signaling, such as Radio Resource Control (RRC) signaling, in which case the scheduling occurs at time instances and with parameters instructed by the higher layer signaling.
A NodeB may also transmit multiple types of RSs including a UE-Common RS (CRS), a Channel State Information RS (CSI-RS), and a DeModulation RS (DMRS). A CRS is transmitted over substantially an entire DL system BandWidth (BW) and can be used by all UEs to demodulate data, control signals, or perform measurements. To reduce an overhead associated with a CRS, a NodeB may transmit a CSI-RS with a smaller density in a time and/or frequency domain than a CRS for UEs to perform measurements, and transmit a DMRS only in a BW of a respective PDSCH. A UE can use a DMRS to demodulate information in a PDSCH.
FIG. 1 illustrates a conventional transmission structure for a DL Transmission Time Interval (TTI).
Referring to FIG. 1, a DL TTI includes one subframe 110 which includes two slots 120 and a total of NsymbDL symbols for transmitting data information, DCI, or RS. First MsymbDL subframe symbols are used to transmit PDCCHs and other control channels (not shown) 130. Remaining NsymbDL−MsymbDL subframe symbols are primarily used to transmit PDSCHs 140. A transmission BW includes frequency resource units referred to as Resource Blocks (RBs). Each RB includes NscRB sub-carriers, or Resource Elements (REs), and a UE is allocated MPDSCH RBs for a total of MscPDSCH=MPDSCH·NscRB REs for a PDSCH transmission BW. An allocation of one RB in a frequency domain and of one slot and two slots (one subframe) in a time domain will be referred to as a Physical RB (PRB) and a PRB pair, respectively. Some REs in some symbols contain CRS 150, CSI-RS or DMRS.
DCI can serve several purposes. A DCI format in a respective PDCCH may schedule a PDSCH or a PUSCH providing data or control information to or from a UE, respectively. Another DCI format in a respective PDCCH may schedule a PDSCH providing System Information (SI) to a group of UEs for network configuration parameters, a response to a Random Access (RA) by UEs, or paging information, for example. Another DCI format may provide Transmission Power Control (TPC) commands to a group of UEs for SPS transmissions in respective PUSCHs or PUCCHs.
A DCI format includes Cyclic Redundancy Check (CRC) bits in order for a UE to confirm a correct detection. The DCI format type is identified by a Radio Network Temporary Identifier (RNTI) that scrambles the CRC bits. The RNTI is a Cell RNTI (C-RNTI) for a DCI format scheduling a PDSCH or a PUSCH to a single UE. The RNTI is an SI-RNTI for a DCI format scheduling a PDSCH conveying SI to a group of UEs. The RNTI is an RA-RNTI for a DCI format scheduling a PDSCH providing a response to a RA from a group of UEs. The RNTI is a P-RNTI for a DCI format scheduling a PDSCH paging a group of UEs. The RNTI is a TPC-RNTI for a DCI format providing TPC commands to a group of UEs. Each RNTI type is configured to a UE through higher layer signaling from a NodeB (and the C-RNTI is unique for each UE).
FIG. 2 illustrates a conventional encoding and transmission process for a DCI format at a NodeB transmitter.
Referring to FIG. 2, an RNTI of a DCI format masks a CRC of a codeword in order to enable a UE to identify a DCI format type. A CRC 220 of (non-coded) DCI format bits 210 is computed and is subsequently masked 230 using an eXclusive OR (XOR) operation between CRC and RNTI bits 240, which is XOR(0,0)=0, XOR(0,1)=1, XOR(1,0)=1, XOR(1,1)=0. A masked CRC is then appended to DCI format bits 250, channel coding is performed 260 such as by using a convolutional code, rate matching 270 is performed to allocated resources, and modulation 280 and transmission of a control signal 290 are performed by interleaving. For example, both a CRC and an RNTI include 16 bits.
FIG. 3 illustrates a conventional reception and decoding process for a DCI format at a UE receiver.
Referring to FIG. 3, a received control signal in step 310 is demodulated and resulting bits are de-interleaved in step 320, a rate matching applied at a NodeB transmitter is restored in step 330, and control information is subsequently decoded 340. DCI format bits in step 360 are then obtained after extracting CRC bits in step 350 which are then de-masked in step 370 through an XOR operation with an RNTI in step 380. A UE then performs a CRC test in step 390. If a CRC test passes, a UE considers the DCI format to be valid and determines parameters for PDSCH reception or PUSCH transmission. If a CRC test does not pass, a UE disregards a presumed DCI format.
A NodeB separately codes and transmits a DCI format in a respective PDCCH. To avoid a first PDCCH transmission blocking a second PDCCH transmission, a location of each PDCCH in a DL control region is not unique. Consequently, a UE needs to perform multiple decoding operations per subframe to determine whether there is a PDCCH intended for the UE. REs carrying a PDCCH are grouped into Control Channel Elements (CCEs) in a logical domain. CCE aggregation levels may include, for example, 1, 2, 4, and 8 CCEs.
FIG. 4 illustrates a conventional transmission process of DCI formats in respective PDCCHs.
Referring to FIG. 4, encoded DCI format bits are mapped to PDCCH CCEs in a logical domain. A first 4 CCEs (L=4), CCE1 401, CCE2 402, CCE3 403, and CCE4 404 are used to transmit a PDCCH to UE1. The following 2 CCEs (L=2), CCE5 411 and CCE6 412, are used to transmit a PDCCH to UE2. The following 2 CCEs (L=2), CCE7 421 and CCE8 422, are used to transmit a PDCCH to UE3. A last CCE (L=1), CCE9 431, is used to transmit a PDCCH to UE4. DCI format bits are scrambled in step 440 by a binary scrambling code and are subsequently modulated in step 450. Each CCE is further divided into Resource Element Groups (REGs). For example, a CCE consisting of 36 REs is divided into 9 REGs, each consisting of 4 REs. Interleaving in step 460, for example block interleaving, is applied among REGs which, assuming Quadrature Phase Shift Keying (QPSK) modulation for a PDCCH, include blocks of 4 QPSK symbols. A resulting series of QPSK symbols is shifted by J symbols in step 470, and each QPSK symbol is mapped to an RE in step 480 in a control region of a DL subframe. Therefore, in addition to the CRS, 491 and 492, and other control channels (not shown), REs in a DL control region contain QPSK symbols corresponding to DCI format for UE1 494, UE2 495, UE3 496, and UE4 497.
For a PDCCH decoding process, a UE may determine a search space for candidate PDCCH transmissions after it restores CCEs in a logical domain according to a UE-common set of CCEs (Common Search Space or CSS) and according to a UE-dedicated set of CCEs (UE-Dedicated Search Space or UE-DSS). A CSS may include first C CCEs in a logical domain, which is used to transmit PDCCHs for DCI formats associated with UE-common control information and use a SI-RNTI, a P-RNTI, a TPC-RNTI, to scramble respective CRCs. A UE-DSS includes remaining CCEs, which are used to transmit PDCCHs for DCI formats associated with UE-specific control information and use respective C-RNTIs to scramble respective CRCs.
CCEs of a UE-DSS is determined according to a pseudo-random function having as inputs UE-common parameters, such as a subframe number or a total number of CCEs in a subframe, and UE-specific parameters such as a C-RNTI. For example, for CCE aggregation level of Lε{1, 2, 4, 8} CCEs, the CCEs corresponding to PDCCH candidate m are given byCCEs for PDCCH candidate m=L·{(Yk+m)mod └NCCE,k/L┘}+i   Equation 1where NCCE,k is a total number of CCEs in subframe k, i=0, . . . , L−1, m=0, . . . , MC(L)−1, and MC(L) is a number of PDCCH candidates to monitor in a UE-DSS. Values of MC(L) for Lε{1, 2, 4, 8} are, respectively, {6, 6, 2, 2}, for example. For a UE-DSS, Yk=(A·Yk-1)mod D where Y−1=C−RNTI≠0, A=39827 and D=65537. For a CSS, Yk=0.
A conventional DL control region may occupy a maximum of MsymbDL=3 subframe symbols and a PDCCH is transmitted substantially over an entire DL BW. Consequently, network functionalities such as extended PDCCH capacity in a subframe and PDCCH interference coordination in a frequency domain, which are needed in several cases, cannot be supported. One such case is a use of Remote Radio Heads (RRHs) in a network where a UE may receive DL signals either from a macro-NodeB or from an RRH. If RRHs and a macro-NodeB share a same cell identity, cell-splitting gains do not exist and expanded PDCCH capacity is needed to accommodate PDCCH transmissions from both a macro-NodeB and RRHs. Another case is for heterogeneous networks where DL signals from a pico-NodeB experience strong interference from DL signals from a macro-NodeB, and interference coordination in a frequency domain among NodeBs is needed.
A direct extension of a conventional DL control region size to more than MsymbDL=3 subframe symbols is not possible at least due to a need for support of conventional UEs, which cannot be aware or support such extension. An alternative is to support DL control signaling in a PDSCH region by using individual PRB pairs. A PDCCH transmitted in one or more PRB pairs of a conventional PDSCH region will be referred to as Enhanced PDCCH (EPDCCH).
FIG. 5 illustrates a conventional EPDCCH transmission structure in a DL subframe.
Referring to FIG. 5, although EPDCCH transmissions start immediately after PDCCH transmissions 510 and are over all remaining subframe symbols, they may instead always start at a fixed location, such as the fourth subframe symbol, and may extend over a part of the remaining subframe symbols. EPDCCH transmissions occur in four PRB pairs, 520, 530, 540, and 550 while the remaining PRB pairs are used for PDSCH transmissions 560, 562, 564, 566, 568.
A UE can be configured by higher layer signaling from a NodeB for one or more sets of PRB pairs that may convey EPDCCHs. The transmission of an EPDCCH to a UE is in one or a few PRB pairs, if a NodeB has accurate CSI for the UE and can perform Frequency Domain Scheduling (FDS) or beam-forming. This transmission is in many PRB pairs, possibly also using transmitter antenna diversity, if accurate CSI per PRB pair is not available at the NodeB. An EPDCCH transmission over one or a few PRB pairs will be referred to as localized or non-interleaved, while an EPDCCH transmission over many PRB pairs will be referred to as distributed or interleaved.
An exact EPDCCH search space design is not material to the present invention and may follow same principles as a PDCCH search space design. An EPDCCH includes respective CCEs, referred to as ECCEs, and a number of EPDCCH candidates exist for each possible ECCE aggregation level LE. For example, LEε{1, 2, 4, 8} ECCEs exist for localized EPDCCHs and LEε{1, 2, 4, 8, 16} ECCEs exist for distributed EPDCCHs. An ECCE may or may not have the same size as a conventional CCE.
A number of EPDCCH REs per PRB pair varies depending on a size of a conventional DL control region, defined by a number of MsymbDL subframe symbols in FIG. 1, on a number of CSI-RS REs and CRS REs, for example. This variation can be addressed either by maintaining a same ECCE size and possibly having a variable number of ECCEs per PRB pair in different subframes (and possibly also having some REs that cannot be allocated to an ECCE) or by maintaining a same number of ECCEs per PRB pair and having a variable ECCE size.
An ECCE size is defined by a respective number of REs available for transmitting an EPDCCH (excluding REs used to transmit other signals in a PRB pair), and is different than a fixed, maximum ECCE size. A maximum ECCE size is obtained by assuming that no signals, other than DMRS associated with demodulation of EPDCCHs, are transmitted in PRB pairs used to transmit EPDCCHs. Then, for a PRB pair including NscRB=12 REs, NsymbDL=14 subframe symbols, and 24 REs for DMRS transmission, there are NscRB·NsymbDL−24=144 REs available for transmitting EPDCCHs, and the maximum ECCE size is 36 REs for 4 ECCEs per PRB pair.
FIG. 6 illustrates conventional variations in an ECCE size per subframe assuming four ECCEs per PRB pair.
Referring to FIG. 6, in a first realization for a number of REs to transmit EPDCCHs 610, a conventional DL control region spans a first three subframe symbols 620 and there is a first number of DMRS REs 630, CSI-RS REs 632, and CRS REs 634. An ECCE size is 21 REs for 4 ECCEs per PRB pair. In a second realization for a number of REs to transmit EPDCCHs 640, a conventional DL control region spans a first one subframe symbol 650, and there is a second number of DMRS REs 660 and CRS REs 662 (no CSI-RS REs). An ECCE size is 30 REs for 4 ECCEs per PRB pair, or about 43% more than in the first realization. Larger variations in an ECCE size may also exist.
As an ECCE size may vary per subframe, and a minimum ECCE aggregation level required to reliably detect an EPDCCH may also vary. A threshold for an ECCE size TRE is defined, and ECCE aggregation levels are twice the aggregation levels for when an ECCE size is greater than or equal to TRE, when an ECCE size is less than TRE. For example, if LEε{1, 2, 4, 8} are ECCE aggregation levels for transmitting a distributed EPDCCH with an ECCE size greater than or equal to TRE, then LEε{2, 4, 8, 16} are ECCE aggregation levels for transmitting a distributed EPDCCH with an ECCE size less than TRE.
Using a single TRE value fails to properly address a need for a UE to detect different EPDCCHs conveying DCI formats with different information payloads. For example, a first DCI format scheduling a PUSCH may have a payload of 43 bits while a second DCI format scheduling a PDSCH may have a payload of 58 bits. Then, for QPSK and for TRF=26, even though an aggregation level of one ECCE may sufficiently convey the first DCI format, as a respective maximum code rate is 0.83, an aggregation level of one ECCE cannot sufficiently convey the second DCI format as a respective maximum code rate is 1.12 and TRE=35 would be required for a maximum code rate of about 0.83 for the second DCI format.
Using a single TRE value also fails to account for a variable modulation scheme to transmit an EPDCCH. The modulation scheme is one of the components determining a respective code rate for transmitting an information payload of a DCI format in an EPDCCH for an ECCE aggregation level.
Using a single TRE value further fails to account for variations in an information payload of a DCI format according to a presence or absence of configurable information fields, variations of resource allocation, or other information fields according to a DL or UL operating bandwidth. A UE can determine a DL or UL operating bandwidth by receiving system information transmitted by a NodeB.
Therefore, a need exists in the art to define multiple thresholds of ECCE sizes, each threshold corresponding to one or more DCI formats a UE attempts to detect, for adjusting respective ECCE aggregation levels for EPDCCH candidates depending on whether an ECCE size is less than a threshold or greater than or equal to a threshold.
A need exists in the art to define a threshold of an ECCE size for adjusting respective ECCE aggregation levels for EPDCCH candidates depending on a respective modulation scheme.
A need exists in the art to adjust ECCE aggregation levels for EPDCCH candidates for different DCI formats depending on a maximum bandwidth that can be scheduled, and on the DCI format.
A need exists in the art to adjust ECCE aggregation levels for EPDCCH candidates for different DCI formats depending on a code rate for a respective DCI format.